1. Field of the Invention
The present invention relates to methods for the recovery of precious metals from host materials, using pressure oxidation.
2. Description of the Related Art
Traditionally, gold is not included in the group of meals known as the platinum group metals which include platinum and palladium, rhodium, ruthenium, osmium and iridium. For the sake of brevity, the term `precious metals` as used herein will refer to both gold and the platinum group metals. Typically, these precious metals are present in host materials along with other metals such as the base metals copper and nickel, in varying concentrations. These host materials are normally treated by grinding and flotation to produce a concentrate, which is then smelted. The constituent precious and base metals typically report to a matte phase during smelting. The matte phase is then processed by a variety of well established techniques to separate and recover the individual constituents in substantially pure form.
The matte typically undergoes oxidative pressure leaching at temperatures of 130 to 150 degrees Celsius which leaches the base metal component of the matte leaving the platinum group metals in the residue. After separating the solution from the residue, the residue is then typically leached, in atmospheric conditions, with concentrated hydrochloric acid and chlorine gas as the oxidant to dissolve the platinum group metals.
In certain cases, when the concentrations of valuable base metals (such as copper and nickel for example) are low, or when the concentrations of metals that are deleterious to smelting (such as arsenic or bismuth) are high, it is not economical to process the concentrates by smelting. In these instances, it can be advantageous to process the concentrate directly by a pressure leach process to leach the base metals.
Typically, the solid phase leaving the pressure leach step retains the precious metals while the copper and nickel transfer to the leach solution, for further processing by various hydrometallurgical procedures well known to the art. In this case, the precious metal constituent is recovered from the solid phase by relatively complex and expensive procedures, including leaching by chlorination or pressure cyanidation, followed by precious metal recovery by precipitation, solvent extractions and ion exchange techniques well known to the art. Though these processing routes may prove satisfactory in some cases, there are many host materials in which either the leach efficiency or the concentration of precious metals is insufficient for this relatively complex metallurgical flowsheet to be economically viable.
It is an object of the present invention to provide an improved technique for the recovery of precious metals.